1. Field of the Invention
The present invention generally relates to a common rail such as a high-pressure fuel branch pipe or a block rail in an accumulated-pressure fuel injection system for a diesel internal combustion engine.
2. Description of the Prior Arts
A number of constructions have heretofore been known in the field of this kind of diesel engine fuel injection pipe. In the construction shown in FIG. 6 by way of example, a main pipe rail 1 has a circulating passage 1-1 of circular cross section formed in a peripheral portion of the interior of the main pipe rail 1 as well as a branch hole 1-2 which communicates with the circulating passage 1-1, and the branch hole 1-2 is outwardly opened to form a pressure-receiving seat surface 1-3. A ring-shaped connecting fitting 3 is used to surround the outer circumferential portion of the main pipe rail 1 in the vicinity of the pressure-receiving seat surface 1-3. A branch pipe 2 which serves as a branch connector has a connecting head 2-2 at one end, and the connecting head 2-2 is formed into, for example, a tapered conical enlarged-diameter shape by buckling. A pressure-applying seat surface 2-3 formed by the connecting head 2-2 is brought into engagement with the pressure-receiving seat surface 1-3, and a nut 4 which is fitted on the branch pipe 2 by a sleeve washer 5 in advance is screwed into a threaded wall portion 3-1 which is formed in the connecting fitting 3 in such a manner as to project radially outwardly from the main pipe rail 1. The branch pipe 2 is connected to the main pipe rail 1 in the state of being fastened by the pressure applied to the neck of the connecting head 2-2 due to the screwing of the nut 4 into the threaded wall portion 3-1. In the known construction shown in FIG. 7 or 8, instead of the ring-shaped connecting fitting 3, a pipe-shaped sleeve nipple 3a or 3b is directly fitted to the outer peripheral wall of the main pipe rail 1 by a method of screwing a projecting portion into a recessed portion or by welding so that the pipe-shaped sleeve nipple 3a or 3b projects radially outwardly from the main pipe rail 1. The pressure-applying seat surface 2-3 formed by the connecting head 2-2 of the branch pipe 2 is brought into engagement with the pressure-receiving seat surface 1-3 of the main pipe rail 1, and a nut 4 which is screwed into the pipe-shaped sleeve nipple 3a or 3b is fastened to connect the branch pipe 2 to the main pipe rail 1. In the known construction shown in FIG. 9, a boss 3c is formed integrally with the main pipe rail 1 of a common rail, and the pressure-applying seat surface 2-3 formed by the connecting head 2-2 of the branch pipe 2 is brought into engagement with the pressure-receiving seat surface 1-3 of the main pipe rail 1. A box nut 6 which is screwed on a threaded portion 3-2 provided around the outer circumferential surface of the boss 3c is fastened to connect the branch pipe 2 to the main pipe rail 1. A block rail type common rail (not shown) is also known.
However, in any of the prior art common rails, a large stress occurs in an inner circumferential edge portion P of the lower end of the branch hole 1-2 owing to the inner pressure of the main pipe rail 1 and an axial force applied to the pressure-receiving seat surface 1-3 by the pressure of the connecting head 2-2 of the branch pipe 2 which serves as a branch connector. Cracks easily occur from the lower-end inner circumferential edge portion P, and there is a possibility that the cracks causes leakage of fuel. In addition, a nonmetal inclusion contained in a parent metal may be exposed on a surface by cutting. This nonmetal inclusion is mainly made of an oxide such as Al2O3 or CaO, and is extremely high in hardness and extremely small in elongation compared to the parent metal and is, in addition, weak in bonding force to the parent metal. If such a nonmetal inclusion exists in the inner circumferential edge portion P of the lower end of the branch hole 1-2, stress concentration occurs as in the above-described case and causes a fatigue failure, so that the fatigue strength of the main pipe rain 1 is lowered. in addition, if the nonmetal inclusion exists in a surface portion immediately below the surface of the parent metal, the lowering of the fatigue strength. is similarly Incurred. The lowering of the fatigue strength due to the nonmetal inclusion is considered to be caused by an increase in stress concentration due to the difference in hardness or elongation between the parent metal and the nonmetal inclusion.
The invention has been made to solve the above-described problems of the prior art, and provides a common rail for a diesel engine in which fatigue strength against inner pressure can be increased by lowering the extent of concentration of stress which occurs in the portions of intersections of branch holes which include the inner peripheral edges of the lower ends of the respective branch holes 1-2 and a circulating passage of the main pipe rail, or in the branch holes, the inner circumferential surface of the main pipe rail or the like.
The common rail for a diesel engine according to the invention includes a main pipe rail having a circulating passage extending in its inside in the axial direction thereof, branch holes formed in a peripheral wall portion of the main pipe rail, and branch connectors connected to the respective branch holes integrally or via separate connecting members, and a Ni-diffused fatigue strength reinforcing layer which is formed by heating a Ni layer prepared in advance by plating of pure Ni or a Ni-base alloy such as Nixe2x80x94P is formed in at least a portion of an inner circumferential surface of the common rail.
In addition, portions in which to form the Ni-diffused fatigue strength reinforcing layer which is formed by heating the Ni layer plated in advance may be the branch holes and the portions of intersections of the branch holes and the circulating passage of the main pipe rail, or the circulating passage of the main pipe rail, or the entire circumferential surface of the common rail.
In the common rail for a diesel engine, in the case where the Ni-diffused fatigue strength reinforcing layer which is formed by heating the Ni layer plated in advance is formed in the portion of the surface or a surface portion of a parent metal where a nonmetal inclusion is present, the fatigue strength reinforcing layer formed by heating this Ni layer is larger in hardness and smaller in elongation than the parent metal, whereby the difference in hardness or elongation between a steel surface and the nonmetal inclusion in the surface or the surface layer of the parent metal becomes small. In the case where this fatigue strength reinforcing layer is formed in, for example, the portions of intersections of the branch holes and the circulating passage of the main pipe rail, the degree of concentration of fatigue stress which occurs in the portions of intersections of the branch holes and the circulating passage of the main pipe rail is lowered and the maximum value of stress which occurs in the intersection portions is lowered, whereby fatigue strength against inner pressure is improved. Incidentally, the thickness of the fatigue strength reinforcing layer is not limited to a particular value, but 10-30 xcexcm is appropriate for the fatigue strength reinforcing layer to serve its effect and advantage.
Incidentally, each of the portions of intersections of the branch holes and the circulating passage of the main pipe rail over which the Ni-diffused fatigue strength reinforcing layer is formed may be chamfered in an arcuate shape to form an R-chamfered portion made of a curved surface having no edge The cross-sectional shape of this R-chamfered portion may be a shape in which a tapered surface is joined to the inner circumferential surface of the branch hole via a smooth curved surface and the inner circumferential surface of the circulating passage of the main pipe rail is joined the tapered surface via the smooth curved surface, or the shape of a spherical surface, an ellipsoidal surface of revolution a paraboloid of revolution or a hyperboloid revolution.